Thermodynamic evaluation of the phase equilibria and glass-forming ability of the Al-Co-Gd system

The Al-Co-Gd system was thermodynamically assessed by means of the CALPHAD (CALculation of PHAse Diagrams) method. The substitutional model was adopted to describe the thermodynamic functions of solution phases and sublattice models were used to describe the intermetallic phases in the ternary system. A set of self-consistent thermodynamic parameters was obtained. Furthermore, by using Turnbull Gibbs free energy empirical equations, the driving forces for crystallization of the primary crystalline phases from the undercooled liquid alloys were calculated. Combining thermodynamic data with Davies-Uhlmann kinetic formulas, the time-temperature-transformation (TTT) curves were obtained. By comparing critical cooling rates calculated from the TTT curves, the glass-forming ability of seven Al-Co-Gd alloys was evaluated. The results agree with the experimental data, which suggests that the combined thermodynamic and kinetic approach may be an efficient way for the evaluation of glass-forming ability.